Recent studies demonstrated that selenium (Se) reduced prostate cancer incidence in humans and induced apoptosis and growth inhibition of prostate cancer cells in vitro. However, the underlying chemopreventive mechanism(s) of Se remains unclear. Production of superoxide radicals by Se has been considered as one of the potential mechanisms. Our studies showed that Se induced cell apoptosis and growth inhibition with superoxide radical production and up-regulation of p53, p21(Waf1), Bax, and some antioxidant enzymes, suggesting a reduction-oxidation (redox)-mediated effect of Se on the activation of p53. Thus, we hypothesize that Se induces growth inhibition and apoptosis of prostate cancer cells in part by modulating cell redox state resulting in upregulation and activation of p53, Bax, and p21(Waf1). Specific Aim 1 is to determine whether Se has differential redox effects in normal prostate epithelial cells, immortalized epithelial cells, and cancer cells by measuring cellular antioxidant status, levels of reactive oxygen species, and oxidative stress following Se treatment. Specific Aim 2 is to determine whether superoxide radical production plays a major role in redox effects of Se on p53 phosphorylation and activation and subsequent cell growth inhibition and apoptosis and whether mitochondria are the primary targets or pathways for cell apoptosis; this aim will be tested by altering levels of superoxide dismutases (SOD) using gene transfection and siRNA silencing. Levels of superoxide radical and mitochondrial SOD2 will be correlated with cellular effects of Se. Specific Aim 3 is to determine the role of p53 and its target genes Bax and p21(Waf1) in Se-induced apoptosis and cell cycle arrest using gene transfection and siRNA silencing to alter p53 status and suppress Bax and p21(Waf1) pathways. The proposed studies will provide insights into the mechanism(s) and molecular targets of Se in prostate cancer chemoprevention and help in the development of specific and effective dietary intervention strategies for prostate cancer prevention.